Oh, I like the Minute Mysteries!
They're short several-page stories featuring the American Girls that have a mystery component or something to solve at the end. I used to love this kind of thing when I was in elementary and middle schools - my gifted teacher used to do stuff like this with us, too. Some are easy and are good for even my little niece to join in on the fun, but some are pretty tough and require a good amount of reasoning and thought. I'm probably pretty bad at describing things, but I think that they're definately worth getting. If you want to take a look for yourself, try going to your local bookstore and skimming through one of the stories.